1. Field of the Invention
The present invention relates to a semiconductor device and a method of producing the same.
2. Related Arts
When connecting a power metal oxide semiconductor field effect transistor (a power MOSFET) to a load, there are two types of connection, i. e., a high side connection shown in FIG. 28 and a low side connection shown in FIG. 29. In the high side connection, a power MOSFET 41 is disposed between a power supply V.sub.DD and a load 40. In this connection, even if the load 40 itself is short-circuited to the earth, the power MOSFET 41 can control current flowing through the load. Therefore, in many cases, the high side connection is adopted because of high reliability.
Especially, a p-channel power MOSFET is suitable for the high side connection, because the p-channel power MOSFET is turned ON when a gate electric potential is controlled to be negative with respect to a source electric potential. However, the ON resistance of the p-channel power MOSFET is two or three times higher than that of an n-channel power MOSFET if the chip sizes thereof are the same. To obtain low ON resistance of the p-channel power MOSFET, it is effective to increase the chip size of the MOSFET. However, large chip size results in high cost. Because of this, at present, there is a case where the n-channel power MOSFET having low ON resistance is used in the high side connection as shown in FIG. 30. In the case where the n-channel power MOSFET is arranged in the high side connection, it is necessary to employ a voltage rising circuit 42 for raising the gate electric potential of the n-type channel power MOSFET, because the n-type channel power MOSFET is not turned ON unless a gate electric potential is higher than a source electric potential. The provision of such voltage rising circuit 42 results in high cost on its system design and the like. Accordingly, in the case where the n-channel power MOSFET is more advantageous in cost than that of the p-channel power MOSFET having high ON resistance, the n-channel power MOSFET is likely to be employed.
Here, if the p-type channel power MOSFET having low ON resistance is obtained, since the p-channel power MOSFET does not need the voltage rising circuit or the like as shown in FIG. 28, a power MOSFET high side switch having a simple system structure can be realized.
Recently, an n-channel power MOSFET, in which resistance component associated with junction field effect transistor (JFET) parasitically caused in the power MOSFET is lowered by forming a groove in a surface of a silicon substrate, has been proposed. In addition, in this n-channel power MOSFET, a semiconductor fine processing technique is applied to reduce a size of a unit cell of the power MOSFET. As a result, degree of integration is increased, resulting in decrease of the ON resistance.
For example, JP-A-56-96865 and JP-A-60-28271 disclose the n-channel power MOSFET utilizing a local oxidation of silicon (LOCOS) technique. More concretely, a local oxide (LOCOS) layer is formed on a surface of a silicon substrate, and then, double diffusion layers are formed by utilizing the LOCOS layer as a diffusion mask. Thereafter, the LOCOS layer is removed to form a silicon groove, and a gate oxide layer and a gate electrode are formed on the silicon groove.
To form the n-channel power MOSFET, arsenic (As) ions are usually implanted to form a shallow source diffusion layer which is one of the double diffusion layers.
By utilizing the above mentioned technique, the unit cell size of the n-channel power MOSFET is reduced and simultaneously the ON resistance is lowered.
However, in the case where the above mentioned structure is applied to a p-channel power MOSFET, there arise the following problems.
That is to say, as impurities for forming a source diffusion layer of the p-channel power MOSFET, boron (B) ions having a diffusion coefficient larger than that of As ions are utilized in place of As ions.
In general, after forming the source diffusion layer, there exist some heat treatment steps which inevitably cause further diffusion of the source diffusion layer. In particular, in the case of the p-channel power MOSFET with the groove, because the diffusion coefficient of B ions is large, a diffusion depth of B ions becomes deep during the heat treatment steps such as gate oxidation, gate formation, and the like. As a result, a thickness of a channel region becomes small, giving rise to a punch through phenomenon.
To prevent the punch through phenomenon, it is effective to reduce the impurity concentration of boron ions in the source diffusion layer. Accordingly, the diffusion depth of boron ions becomes shallow, resulting in a shallow junction. In this case, however, the contact resistance between the source diffusion layer and a source electrode made of aluminum or the like increases, so that the ON resistance of the p-channel power MOSFET increases as well.